Material handling machines are available in a wide variety of configurations to suit particular applications. Such machines include fork-type lift trucks, front end loaders and many others.
Another type of material handling machine (and the one to which the invention relates), is known as an overhead travelling crane. In a common configuration, such cranes include a pair of bridge girders spanning rather widely spaced railroad type bridge rails. Such rails are suspended above, for example, a factory floor or an outdoor steel handling yard. The girders are supported and propelled by flanged wheels riding atop the rails.
Mounted atop each girder and extending along its length is a trolley rail, atop which is mounted a trolley capable of "traversing" movement, i.e., movement along a line generally normal to the line of movement of the entire crane. The trolley is equipped with at least one hoist drive and a load-hoisting hook (or other load-handling device) attached below a bottom block for moving loads from place to place. So configured, the crane is capable of lifting a load from any location on a factory floor, for example, and moving it to any other location.
A factor considered by designers of overhead travelling cranes is the possibility of the bottom block being raised to an elevation at which it strikes the solid undercarriage of the trolley or even "wraps" on the hoist drum. In either event, there is a substantial risk of breaking the stranded rope-like steel cables by which the bottom block is attached to the rotating hoist drum. If a cable breaks, there is a chance that the load will be uncontrollably dropped.
To help guard against that eventuality, crane designers have employed a control circuit limit switch and a power circuit limit switch actuated in one of the ways described below. If the bottom block reaches a certain elevation, the control limit switch is tripped. Such limit switch tripping disables the control circuit or, in the alternative, "reconfigures" the control circuit in such a way that the hoist drive controller causes the rate of bottom block ascent to slow markedly.
If the operator fails to stop the hoist drive or if the control limit switch malfunctions for some reason, the bottom block continues its upward movement and trips the power limit switch. This opens the "raise" power connections to the hoist drive motor and stops bottom block movement before such block strikes the trolley undercarriage or otherwise reaches an abnormal position.
One type of control limit switch has a lever-like counterweight assembly attached to the switch shaft. Such assembly is biased to an operating position by a weight suspended from the assembly by a cable. When tension on such cable is relieved by inadvertently hoisting the bottom block until it lifts the suspended weight, the counterweight "trips" the switch.
Similarly, the power circuit limit switch has a heavy block-shaped weight suspended from one end of a limit switch arm, the other end of which has a counterweight. The torque produced by the suspended weight is greater than that produced by the counterweight and the suspended weight retains the power switch in the operative position. On the other hand, when the suspended weight is lifted by the ascending bottom block, the counterweight "takes over" and trips the power switch.
While these arrangements have been generally satisfactory, they are less than completely so in certain situations. For example, if both the power limit switch and the control limit switch are of the suspended-weight type, the weights should be generously spaced apart laterally so as to avoid interfering with one another. And a cramped installation may not afford the luxury of such lateral spacing.
Yet another characteristic of known arrangements is that they lack easy-to-use means for adjustment of the elevation at which one weight is suspended relative to the other. Another characteristic is that in general they are limited to applications involving a single control switch.
A dual weight assembly which addresses the aforementioned characteristics of known arrangements would be an important advance in the art.